Process for producing grain-oriented electrical steel strip and grain-oriented electrical steel strip obtained according to said process

ABSTRACT

A process for producing grain-oriented electrical steel strip by means of thin slab continuous casting, comprising the following process steps: a) smelting a steel, b) continuously casting the smelt by thin slab continuous casting, c′) heating up the thin slabs and subjecting the slabs to homogenization annealing at a maximum temperature of 1,250° C., d) heating to a temperature between 1.350° C. and 1.380° C., e) continuously hot rolling the thin slabs to form a hot-rolled strip, f) cooling and reeling the hot-rolled strip to form a coil, g) annealing the hot-rolled strip after reeling and prior to a subsequent cold rolling step, h) cold rolling the hot-rolled strip to the nominal usable thickness, i) subjecting the cold-rolled strip to recrystallization, decarburization and nitridation annealing, j) applying an annealing separator (non-stick layer) to the strip surface of the cold-rolled strip, k) subjecting the cold-rolled strip to secondary recrystallization annealing, forming a finished steel strip having a pronounced Goss texture, and l) stress-free annealing the finished steel strip, which has been coated with an insulating layer, provides an improved process for producing grain-oriented electrical steel strip by means of thin slab continuous casting. This is achieved in that the recrystallization, decarburization and nitridation annealing of the cold-rolled strip in process step h) comprises a decarburization annealing phase and a subsequent nitridation annealing phase, with an intermediate reduction annealing phase being interposed between the decarburization annealing phase and the nitridation annealing phase, and carried out at a temperature ranging from 820° C.-890° C., for a maximum period of 40 seconds, with a dry, gaseous annealing atmosphere, which contains nitrogen (N 2 ) and hydrogen (H 2 ) and acts on the cold-rolled strip, and which has a water vapor/hydrogen partial pressure ratio pH 2 O/pH 2  of less than 0.10 and wherein a cold-rolled strip is obtained, which primary recrystallized grains have a circle equivalent mean size (diameter) between 22 μm and 25 μm.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is directed to a process for producing grain-orientedelectrical steel strip by means of thin slab continuous casting, saidprocess comprising the process steps of:

a) smelting a steel with a smelt which, particularly after secondarymetallurgical treatment, contains, in addition to iron (Fe) andunavoidable impurities, Si: 2.50-4.00 wt %, C: 0.030-0.100 wt %, Mn:0.060-0.300 wt %, Cu: 0.100-0.300 wt %, Alsl: 0.020-0.040 wt %, Sn:0.050-0.150 wt %, S: <100 ppm, N: <100 ppm, and one or more elementsfrom the group comprising Cr, V, Ni, Mo and Nb,b) continuously casting the smelt by thin slab continuous castingwithout exposure of the strand to inert gas to form a strand having athickness of 50-120 mm, and dividing the strand into thin slabs,c) carrying out a homogenization annealing comprising the steps ofc′) heating up the thin slabs, preferably in a linear furnace, to atemperature above 1,050° C. and subjecting the slabs to an annealing ata maximum temperature of 1,250° C., preferably a maximum temperature of1,200° C., in particular, a maximum temperature of 1,150° C., andd) feeding the thin slabs to an inductive heating device, particularly ahigh frequency inductive heating device, in which the thin slabs,particularly while passing are directly before the first hot rollingpass, at least for some seconds heated up to a temperature above theprevious homogenization temperature of step c′), which temperature iswithin the temperature range of 1.350° C.-1.380° C., especially of1.355° C.-1.370° C., and is particularly 1.360° C.,e) continuously hot rolling the thin slabs in a preferably linear,multiple-stand hot rolling train to form a hot-rolled strip having athickness of 1.8 mm-3.0 mm,f) cooling and reeling the hot-rolled strip at a reeling temperaturebelow 650° C. to form a coil,g) annealing the hot steel strip after reeling and prior to subsequentcold rolling, at a temperature of between 920° C. and 1,150° C.,h) cold rolling the hot steel strip, preferably on a reversible stand,in a single process step in more than three passes, to a cold-rolledstrip having a final thickness of 0.15 mm-0.40 mm,i) subjecting the cold-rolled strip to recrystallization,decarburization and nitridation annealing,j) applying an annealing separator (non-stick layer) containingprimarily MgO to the strip surface of the cold-rolled strip which hasundergone recrystallization, decarburization and nitridation annealing,k) subjecting the cold-rolled strip which has been coated with theannealing separator to secondary recrystallization annealing byhigh-temperature annealing in a bell-type furnace, at a temperatureof >1,150° C., forming a finished steel strip having a pronounced Gosstexture, andl) coating the finished steel strip which has undergone secondaryrecrystallization annealing with an electrically insulating layer, andthen stress-free annealing or stress-relief annealing the coatedfinished steel strip.

The invention is further directed to a grain-oriented electrical steelstrip that is obtained by said process.

2. Description of the Prior Art

The grain-oriented electrical steel strip produced by said process isintended for use in transformers. The material of the grain-orientedelectrical steel strip is characterized by a particularly sharp{110}<001> texture (Miller indices), which has a easy magnetizationdirection parallel to the rolling direction. A method for forming such atexture was first described by N. P. Goss, and therefore, such texturesare generally referred to as “Goss texture”. The Goss texture is formedby selective, anomalous grain growth, also referred to as secondaryrecrystallization. In this process, the normal, natural tendency of ametallic matrix toward grain enlargement is suppressed by the presenceof grain growth inhibitors, also referred to as inhibitors or aninhibitor phase. An inhibitor phase consists of very fine and optimallyhomogeneously distributed particles of one or more second phases. Theseparticles have a natural interfacial energy on their boundary surfacewith the matrix, which inhibits the movement of a grain boundary beyondsaid boundary surface because the further savings of interfacial energyis minimized throughout the system. Such an inhibitor phase is ofcentral significance to the development of the Goss texture andtherefore to the magnetic properties that can be achieved in such amaterial. It is critical in this process to achieve a homogeneousdistribution of a very large number of very small particles, which ismore advantageous than a small number of coarser particles. Since thenumber of precipitated particles cannot be determined throughexperimentation, their size is used as an indication of their efficacy.It is thus assumed that the particles of the inhibitor phase should notbe substantially larger than 100 nm, on average.

In U.S. Pat. No. 1,965,559 A, N. P. Goss describes a process in which agrain-oriented electrical steel strip (silicon steel) is produced byheating up a steel strip, subjecting said strip to a first cold rollingstep, and then subjecting the strip to further heat treatment followedby a second cold rolling step.

Also known in practice are processes in which manganese(II) sulfite(MnS) is used as the inhibitor. The slabs produced by block casting orcontinuous casting are heated to very high temperatures, close to 1,400°C., in order to bring primary, coarse MnS precipitates back intosolution. This diluted MnS is then precipitated finely dispersed duringthe hot working process. Since the hot-rolled strip thus producedalready has the necessary grain growth inhibition this is referred to asinherent inhibition.

However, the grain growth inhibiting effect of the MnS phase is limited,so that, assuming customary hot-rolled strip thicknesses of, e.g., 2.30mm, at least two cold rolling stages are required to bring the steelstrip to its nominal usable thickness, with an intermediaterecrystallization annealing being performed between the individual coldrolling stages. Moreover, material that is inhibited by manganese(II)sulfite can achieve only a limited texture sharpness, in which the Gossposition is scattered on average 7° around the ideal position. Thistexture sharpness is reflected, in the magnetic polarization at a fieldstrength of 800 A/m, which is only rarely able to exceed values of 1.86T. Such material is traditionally referred to as Conventional GrainOriented or CGO.

The traditional production process, proceeding from the hot-rolledstrip, further comprises a two-stage cold working process in which anintermediate, continuous recrystallization annealing step is performedbetween the two stages. Prior to the first cold rolling stage, acontinuous hot strip annealing step is optionally performed, and isfrequently combined with the essential hot strip pickling. The last coldworking step is traditionally followed by a continuous recrystallizationannealing step. This annealing step also removes the carbon from thesteel strip below the magnetic aging limit, which is determined by themaximum carbon content that is soluble in ferrite, or approximately 30ppm C in a composition of Fe with 3 wt % Si. (Carbon is essentialbecause it establishes the correct microstructure during hot rolling.)The recrystallized microstructure of the steel strip that has beenreduced to its nominal usable thickness represents the starting basisfor the subsequent step of secondary recrystallization. This secondaryrecrystallization is accomplished by high temperature annealing in abell-type furnace. Before the coiled rings (coils) are placed in thebell-type annealing furnace, the surface of the steel strip must beprovided with a non-stick layer. An aqueous slurry of magnesium oxide(MgO) is usually used for this purpose. Once the magnetically desirableGoss texture has formed during the high temperature annealing in abell-type furnace, the outer shape of the steel strip is furtherimproved, and an electrically insulating layer is applied to the twoopposing, large-area, wide surfaces of the strip. This is carried out ina continuous annealing furnace.

SU 688527 A1 discloses a production process which likewise involves atwo-stage cold rolling process with a continuous recrystallization stripannealing step between the two stages. However, during this intermediateannealing stage, the strip is also simultaneously decarburized. This hasthe advantage that, after the final cold rolling to the nominal usablethickness, no further continuous strip annealing step is required. Thestrip is simply coated with the non-stick layer (usually MgO) and thenfed directly to a high-temperature bell-type annealing furnace. However,the microstructure of the resulting strip is not recrystallized, but isinstead as-rolled. As a result, during the very gradual heating of thesteel strip during annealing in a bell-type furnace, a microstructuralrecovery is first achieved, followed by a primary recrystallization, andthen secondary grain growth, which causes the formation of the Gosstexture. This process offers the advantages of relatively cost-effectiveand reliable production. However, it carries with it the disadvantagethat it can achieve magnetic values only at the level of CGO material,and not those of High permeability Grain Oriented or HGO material.

In U.S. Pat. No. 3,159,511 A, Taguchi describes a process for producinggrain-oriented electrical steel strip by which improved texturesharpness can be achieved, with scattering of only approximately 3°around the ideal position. This is achieved by the additionalutilization of aluminum nitride (AlN) as the inhibitor phase, thegrain-growth inhibiting effect of which supplements that of MnS. Thisallows a single-stage cold rolling process to be used. The materialobtained in this manner is referred to as High permeability GrainOriented or HGO. The AlN inhibitors are precipitated into the ferriticmicrostructural regions in their final state during hot rolling.However, increasing the carbon (C) content somewhat over that of CGOmaterial allows the AlN particles, which are located in the austeniticmicrostructural regions in the subsequent hot rolled strip annealing, tobe dissolved out again and then precipitated very finely dispersed in ahighly controlled manner. This can be performed at industrially readilyachievable temperatures in a continuous annealing line, since thesolubility temperature of AlN in austenite of 1,080° C. to 1,140° C. ismuch lower than in ferrite. Despite this double formation of the AlNinhibitor phase (both in ferrite during hot rolling and in austeniteduring continuous hot strip annealing), this is referred to as inherentinhibition since it is produced in the hot strip and the graingrowth-inhibiting foreign phase particles are fully present at the startof the “cold process”.

DE 2 351 141 A1 proposes the use of SbSe as a further inherent inhibitorphase.

All of the aforementioned inherent inhibitors that are produced in thehot strip require very high slab reheating temperatures greater than1,350° C. In addition to requiring substantial energy input and highindustrial expenditure, these temperatures result in a high occurrenceof liquid slag (>1%) due to a relatively low-melting Fe—Si eutecticmixture. In addition to the resulting substantial losses in mass yield,industrial annealing systems are heavily stressed, further adding tocost. Therefore, so-called low heating processes are also used, in whichthe slab reheating temperatures are reduced to less than 1,350° C.,ideally less than 1,250° C. A temperature around 1,250° C. isinteresting because it allows hot-rolled strip for grain-orientedelectrical steel to be produced together with conventional flat-rolledsteel. However, in these processes the inhibitor phase cannot be formedin the hot-rolled strip because the substances used as inhibitorparticles cannot be dissolved out sufficiently at these temperatures toallow them to be re-precipitated finely dispersed in the subsequentprocess.

EP 0 619 376 A1 discloses a low heating process in which low slabreheating temperatures can be achieved with traditional inherentinhibition. In this process, only copper(Cu) sulfide, which has asubstantially lower solubility temperature than MnS or AlN or otherknown inhibitors, is used as inhibitor, with a drastic reduction in theslab preheating temperature. The magnetic characteristics that can beachieved in an electrical steel strip by this process are generallybetween those of CGO and HGO material.

With the low heating process, the inhibitors are not formed until alater stage in the overall production process. The material used in thisprocess particularly contains sufficient free unbonded Aluminum (Al). Byvarious methods of nitridation, the AlN inhibitor phase is formed in thesteel strip, which has been cold rolled to its nominal usable thickness.This form of inhibitor phase is not inherently present in the hot-rolledstrip, and is instead first acquired during a later step of thecold-rolled strip treatment process. Such a process involving acquiredinhibition is described in EP 0 219 611 B1.

EP 0 648 847 B1 and EP 0 947 597 B1 describe mixed forms of inherent andacquired inhibition, in which the slab preheating temperatures are setat values above those of low heating methods, but below the thresholdbeyond which deleterious liquid slag forms. Inherent inhibition canthereby be formed only to a limited extent, and alone would not besufficient to produce satisfactory magnetic characteristics in thefinished material/finished strip. However, this disadvantage can beovercome by combining this process with nitridation treatment, becausethe resulting additional acquired inhibition is enough to achievesufficient total inhibition.

In the process involving acquired inhibition, for industrial reasonsonly AlN is used as an inhibitor in practical applications, because onlynitrogen as an interstitial element has a sufficiently high diffusionspeed in the matrix. Sulfides are not used as acquired inhibitor phases,because sulfur can penetrate into the matrix only via vacancy diffusion,which would be far too slow, even with thermal activation.

In nitridation, nitrogen is injected from the outside through the stripsurface into the matrix, causing AlN particles to form there. This mustoccur over the entire strip cross-section up to the center of the strip,so that the matrix will remain uniformly stabilized until the subsequentsecondary recrystallization. During nitridation, ammonia gas (NH₃) isadded to the annealing atmosphere during continuous annealing treatment.

The above-described processes relate to conventional slab technologywith slab thicknesses significantly greater than 150 mm, typically 210mm-260 mm. Another important development in the history ofgrain-oriented electrical steel strip is the use of so-called thin slabtechnology, as described in EP 1 025 268 B1. The main economic advantageof this technology is that thin slabs, which are understood to be (cast)slabs having a thickness of 30 to 100 mm, typically 60 mm-90 mm, are nolonger cooled to the ambient temperature and later reheated to hightemperatures, but are instead fed at a controlled temperature to alinear homogenization furnace, in which they need only to be (re)heatedsomewhat in order to compensate for temperature losses, and tohomogenize their temperature over the length and width of the strip.Immediately thereafter, these thin slabs are then hot rolled. Inpractical use, this results in substantial cost advantages due to thesavings of energy, and an improved hot-rolled strip edge condition, withthe resulting yield improvement (improvement of physical yield).

Due to the limited thermal resistance of the thin slabs and the need totransport them through a roller hearth furnace, the temperature that canbe reached by heating is limited by the thickness of the slab. Forexample, with a slab thickness of 65 mm for a typical grain-oriented Sisteel, 1,200° C. is the critical upper limit for ensuring sufficientpractical production reliability. For this reason, process routes thatare based on thin slab technology, i.e., thin slab continuous casting,are all essentially low-heating methods. Such processes, in which onlythe use of acquired inhibitors by nitridation treatment is considered,are described in U.S. Pat. No. 8,038,806 B2 and in U.S. Pat. No.8,088,229 B2.

As compared with the hot processing method of casting thick slabsfollowed by a two-stage hot rolling process consisting of hot roughrolling (roughing) and hot finish rolling (finishing), theabove-described thin slab technology or thin slab casting/rollingtechnology based on thin slab continuous casting has the particularfeature that it comprises only one hot working step similar to hotfinish rolling. It has been found, however, that separating the processinto quasi-hot roughing and finishing is expedient, since a period of10-30 seconds for recrystallizing the microstructure is beneficial tothe homogeneity of the hot strip and therefore also to the homogeneityof the final characteristics of the finished product, as is disclosed byWO 2011/063934 A1.

One prior art, which discloses the fundamental and essential processsteps of producing grain-oriented electrical steel strip by means ofthin slab continuous casting is described by EP 1 025 268 B1 and EP 1752 548 A1.

Producing grain-oriented electrical steel strip by means of thin slabcontinuous casting followed by homogenization annealing and hot rollingin line, cold-rolling the strip to its nominal usable thickness, andlater nitridation annealing the strip to introduce an acquired graingrowth inhibitor phase still results in practice in fluctuations in theultimate magnetic characteristics across the length and width of thefinished strip, and as a consequence, a decrease in the quality of thefinished strip.

U.S. Pat. No. 6,432,222 B1 discloses a method for producing grainoriented electrical steel wherein a homogenization annealing is carriedout at a temperature of up to 1.350° C.

The object of the invention is therefore to devise a process that willenable the cost-efficient production of high-grade grain-orientedelectrical steel using thin slab continuous casting systems, and inparticular, to devise a solution that will provide a further improvedprocess for producing grain-oriented electrical steel strip by means ofthin slab continuous casting.

SUMMARY OF THE INVENTION

In a process of the type described in the introductory part, the objectof the invention is attained in that the recrystallization,decarburization and nitridation annealing of the cold-rolled strip inprocess step i)

comprises a decarburization annealing phase which is carried out at astrip temperature ranging from 820° C.-890° C. for a maximum period of150 seconds, using a gaseous annealing atmosphere, in particular moist,which contains nitrogen (N₂) and hydrogen (H₂) and acts on thecold-rolled strip, and which has a water vapor/hydrogen partial pressureratio pH₂O/pH₂ of 0.30 to 0.60,and comprises a subsequent nitridation annealing phase, which is carriedout at a temperature ranging from 850° C.-920° C. for a maximum periodof 50 seconds, using a gaseous annealing atmosphere which containsnitrogen (N₂) and hydrogen (H₂) and acts on the cold-rolled strip, andwhich has a water vapor/hydrogen partial pressure ratio pH₂O/pH₂ of 0.03to 0.07, and an intermediate reduction annealing phase, which is carriedout between the decarburization annealing phase and the nitridationannealing phase, and is carried out at a temperature ranging from 820°C.-890° C. for a maximum period of 40 seconds, using a gaseous annealingatmosphere, in particular dry, which contains nitrogen (N₂) and hydrogen(H₂) and acts on the cold-rolled strip, and which has a watervapor/hydrogen partial pressure ratio pH₂O/pH₂ of less than 0.10 andwherein a cold rolled strip is obtained, which primary recrystallizedgrains have a circle equivalent mean size (diameter) between 22 μm and25 μm.

For a grain-oriented electrical steel strip according to the invention,the object of the invention is attained in that the grain-orientedelectrical steel strip is obtained by above-mentioned process.

The process steps of the process according to the invention can becarried out in a system that performs each of the individual processsteps continuously online, but may also be carried out by firstperforming individual process steps or a group of individual processsteps, and then performing the remaining process steps offline in aseparate system.

According to the invention, therefore, particular emphasis is placed onthe procedural, process engineering design of the process step ofdecarburization and nitridation annealing of the cold-rolled strip, andin this connection particularly on the process-stable formation of thegas-surface reaction.

This process of nitridation is a highly sensitive and failure-pronesurface-gas reaction.

The problem with nitridation is that, prior to this step,decarburization annealing is carried out under necessarily highlyoxidative (moist) gas conditions, whereas nitridation is carried out ina drier annealing atmosphere with lower potential for oxidation. Thehighly oxidative decarburization annealing can therefore form variouslycompact or locally inhomogeneous oxidative barrier layers that willinterfere with the subsequent nitridation. To combat this problem, it isproposed according to the invention to insert an intermediate reductionannealing phase (intermediate reduction zone) in order to correct anylocal superoxidation that have formed during the immediately precedingdecarburization annealing phase, so that the nitridation treatment tofollow directly can be performed homogeneously and reproducibly. Thisintermediate reduction annealing phase is carried out under the sameannealing atmosphere as the decarburization annealing immediatelypreceding it, but at a reduced water vapor/hydrogen partial pressureratio of pH₂O/pH₂<0.10, or ideally <0.05. The intermediate reductionannealing phase lasts a maximum of 40 seconds, preferably 10 to 20seconds. The temperature ranges from 820-890° C. and should ideally beapproximately centered between the optimized and selected temperaturelevels at which the preceding decarburization annealing and thesubsequent nitridation annealing will be performed. This facilitates theprocess in process step i) in terms of systems.

As a result of the inserted intermediate reduction treatment, asatisfactory, homogeneous and reproducible formation of the AlNinhibitor phase in the annealed cold-rolled strip is achieved. Thisallows measures to be provided by which, in addition to the inhibitionacquired during nitridation annealing of the cold-rolled strip, aninherent inhibition can also be carried out or initiated. For thispurpose the invention provides that immediately before the firsthot-rolling of step e), in a previous step d) directly before the firsthot rolling pass the thin slabs are fed to an induction heating device,in particular, a high-frequency induction heating device, in which thethin slabs are heated at least for several seconds, particularly in theproduction flow path, to a temperature of 1,350° C.-1,380° C., which isabove the respective (homogenization) temperature of process step c′).

For grain-oriented electrical steel strip, this results in furtheroptions in terms of process engineering, which leads to improvements inmagnetic product characteristics and the homogenization thereof. Inprinciple, although the maximum through-heating temperature for thinslabs is technically limited due to the limited high temperaturestrength of these formats, so that in principle, only an acquiredinhibition should be feasible, with the option proposed here of heatingthe thin slab material for several seconds to temperatures of up to1,380° C., an inherent (partial) inhibition based on MnS and AlN canadditionally be achieved. The unavoidably low high temperature strengthof the material at such high temperatures is non-problematic, since thetechnical configuration of the material conveyance system can bedesigned and implemented such that each of the slabs that are heated tosuch a temperature is picked up and transported from the first stand ofthe hot working stage or the hot-rolling line. The general problem ofliquid slag formation on the thin slab surface does not arise here asthe temperatures between 1.350° C. and 1.380° C. will be reached onlyfor a short time of a few seconds. But these few seconds are sufficientfor obtaining the dissolution of inhibitor particles.

The inherent inhibition that can be achieved thereby is not sufficientto provide the total inhibition being necessary for an overall processcomprising a single cold-rolling stage, and therefore, an acquiredinhibition must still be provided by nitridation annealing, which, as isknown from the prior art, is achieved by nitriding the strip that hasbeen cold-rolled to its usable thickness. However, the significantadvantage is that the inherent partial inhibition that is formed duringhot-rolling stabilizes the microstructure of the strip for its passagethrough the additional process steps, and prevents any parasitic graingrowth processes.

During the primary-recrystallizing, decarburizing and nitridingannealing process neither texture relations nor oxygen contents arecontrolled. In fact values referring to the texture ratio or the oxygencontent are regulated due to the respective controlling of therespective plant, device or process. It exists no monitoring orcontrolling system which governs or controls the texture ration or theoxygen content. The cold-rolled strip obtained as primary recrystallizedgrains have a circle equivalent mean size (diameter) between 22 μm to 25μm. The expression “casting without exposure of the strand to inert-gas”means a casting without exposure the resulting strand to an inert-gasexposure, wherein a customary and usual protection of the metal streampoured out into the tundish or into the mold may nevertheless exist.

In the embodiment of the invention, the process of nitridation, whichcomprises the highly sensitive and susceptible surface-gas reaction, isfurther positively influenced by adding at least 2 wt % to a maximum of12 wt % ammonia (NH₃), in particular, referred to the total gas flowrate, separately to the annealing atmosphere during the nitridationannealing phase in process step i), and by blowing the ammonia onto thetwo opposing, large-area strip surfaces of the cold-rolled strip. Thisresults in an improvement of the production process and the technicalprocess at this point, since the cold ammonia (NH₃) is blown onto theheated strip as a constituent of the annealing gas, where it decaysdirectly and immediately on the strip surface to nitrogen, hydrogen andwater vapor according to the reaction equation NH₃→N++½ H₂+H₂.

Since the nitridation allows a nitrogen content to be established in theannealed strip, it is advantageous according to a further embodiment ofthe invention that, during the annealing in process step i), whichcomprises the decarburization annealing phase, the intermediatereduction annealing phase, and the nitridation annealing phase, thecold-rolled strip is annealed such that after annealing, the cold-rolledstrip has a total nitrogen content of at least 180 ppm, preferably atleast 250 ppm, in particular, at least 300 ppm.

It is further advantageous for the cold-rolled strip to be heated at thestart of recrystallization annealing in process step i) at a heatingrate of more than 100 K/s, which is likewise proposed by the invention.

In a particularly expedient alloy composition for the smelt to be castin the embodiment of the invention, in the smelt in process step a) theratio of manganese (Mn) to sulfur (S) is greater than 6, preferablygreater than 20, and the ratio of aluminum (Al) to nitrogen (N) isgreater than 4, preferably greater than 10.

It is further expedient according to the invention for the superheatingtemperature of the smelt during casting in process step b) to be lessthan 40K, preferably less than 20 K, in particular, less than 12 K, andfor the reduction in thickness of the strand to be implemented accordingto the “Liquid Core Reduction” method just below the metal mold whilethe core inside the strand is still liquid.

Advantageously, the hot rolling in process step e) is carried out at aninitial rolling temperature during the first working pass of greaterthan 1,150° C., preferably greater than 1,200° C., a final rollingtemperature ranging from 850° C.-980° C., and a final rolling speed ofless than 12 m/s, preferably less than 10 m/s, which is likewiseproposed by the invention.

In a further embodiment, the invention is characterized in that, duringannealing of the hot-rolled strip in process step g), the annealedhot-rolled strip is quenched after annealing at a cooling rate of morethan 25 K/s, preferably more than 30 K/s, in particular, more than 40K/s, particularly preferably a cooling rate ranging from 25 K/s-52 K/s.

It is further expedient in the embodiment of the invention for theworking during cold-rolling in process step h) to be carried out suchthat, during at least one or more of the last three passes, thehot-rolled strip reaches a temperature, generated by the processing heatduring rolling, of at least 180° C. to a maximum of 260° C., for atleast five minutes.

The cold rolling in process step h) can also be carried out in twostages, therefore the invention further proposes that the cold rollingin process step g) be carried out in two stages, wherein the hot-rolledstrip is pickled in a pickling step prior to the first cold-rollingstage, and once the first stage of cold rolling has been completed, thehot-rolled strip is annealed according to process step g). In this case,it is then further advantageous for the thickness of the hot-rolledstrip to be reduced by cold rolling by at least 85% in the second stageof cold rolling.

An annealing atmosphere that is advantageous for secondaryrecrystallization annealing can be achieved according to the inventionby carrying out the secondary recrystallization annealing in processstep k) such that during the heating phase of the high-temperatureannealing in a bell-type furnace, the percentage of nitrogen (N₂) in thegaseous annealing atmosphere, in terms of atomic percent, is greaterthan the percentage of hydrogen (H₂), in terms of atomic percent.

Finally, the invention also provides that, following process step l), inparticular optionally, a process step is performed which effects amagnetic domain refinement of the coated finished steel strip.

Overall, the invention is based on a basic alloy system that iscustomarily used for grain-oriented electrical steel strip and comprisesiron at a proportion of 2 to 6.5 wt % and Si, typically a Si content of3.2 wt %. Other suitable alloy elements are carbon, manganese, copperand aluminum, along with sulfur and nitrogen. The contents of manganesewithin the range of 0.060 to 0.300 wt %, particularly above 0.150 wt %and preferably within the range of 0.160 to 0.300 wt %. The sulfurcontent is set below 100 ppm and preferably below 50 ppm. The smelt iscast to form a strand using a thin slab casting machine without exposureof the strand to inert gas. This strand is then divided into thin slabs,and these thin slabs are subjected to homogenization annealing in acontinuous furnace at a temperature greater than 1,050° C., preferablyat 1,150° C. The thin slabs are then heated up rapidly to a temperatureabove 1,350° C. and up to 1,380° C. using an in line induction heatingapparatus, and immediately thereafter, the thin slabs are hot worked toa hot strip thickness ranging from 1.8 mm-3.0 mm, preferably from1.80-2.30 mm. Once the hot-rolled strip produced in this manner has beenpickled and annealed, it is cold rolled to its nominal usable thicknessranging from 0.15 mm-0.50 mm, preferably ranging from 0.23 mm-0.40 mm,wherein the processing heat, having a temperature range of 180° C. to260° C., is allowed to act on the strip for at least 5 minutes,preferably 6 minutes. The cold-rolled strip thus produced is thenrecrystallized, decarburized and nitrided in a continuous annealingline, during which the nitrogen content is increased to at least 180ppm. Between the partial steps of decarburization and nitridation, anintermediate reduction phase is implemented to adjust the oxidic surfacelayer. During the primary recrystalization, decarburization, andnitridation annealing, the texture ratios and the oxygen content areagain not controlled. Rather, these values are adjusted automatically bycontrol of the respective system and process. There is not any controlor regulating system for controlling the texture ratios or oxygencontent.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A cold-rolled strip will be obtained which primary recrystallized grainshave a circle equivalent mean size (diameter) between 22 μm and 25 μm.

After a non-stick coating (annealing separator), particularly consistingof MgO, is applied, the material is subjected to high-temperatureannealing in a bell-type annealing furnace, at a temperature above1,150° C. and up to 1,200° C., for the purpose of adjusting and formingthe magnetically required Goss texture. An insulating coating is thenapplied, which is followed immediately by continuous stress-reliefannealing. Following inspection, certification and adjustment, theresult is a grain-oriented electrical steel strip in the form of afinished strip ready for use. During carrying out the process, duringthe secondary recrystallization the texture ratios and the oxygencontent are again not controlled. Rather, these values are adjustedautomatically by control of the respective system and process. There isnot any control or regulating system for controlling the texture ratiosor oxygen content.

The chemical composition of the smelt for casting is stated for thefollowing reasons:

Silicon causes an increase in specific electric resistivity andtherefore a decrease in the classic magnetic losses. Below an alloyingdegree of 2 wt %, its use as grain-oriented electrical steel does notmake sense. An alloying percentage above 4 wt % impedes processingtremendously due to the massive brittleness that results. In practicalapplications, Si alloying percentages of 3.15 to 3.30 wt % have provenadvantageous. Beyond even 3.45 wt %, the aforementioned problems withbrittleness are observed.

During high temperature processes carbon causes structuralhomogenization as a result of ferrite-austenite transformation. Carboncontents of between 0.030 and 0.100 wt %, preferably 0.045-0.065 wt %,are generally standard. This effect is intensified with high C contents;however, the decarburization step that is necessary during this processthen requires more time, thereby reducing productivity.

The alloy element manganese generally has a favorable effect on castingand hot working properties. Moreover, a certain Mn content is helpful inreducing wear and tear on refractory material during the liquidmetallurgical treatment steps. Mn contents ranging from 0.060-0.300 wt %have proven advantageous in practice, wherein according to the inventionMn-content is within the range >0.15% by weight, in particular in therange from 0.16-0.3% by weight.

In the process considered here, sulfur is more of a detractive elementand is decreased to contents of less than 100 ppm. The sulfur contentshould preferably be less than 40 ppm. During solidification of thesmelt, MnS particles form, which are retained in the very coarse statein which they are precipitated during solidification of the smeltthroughout the entire process, and are magnetically deleterious in thefinished product. However, reducing the S content will result in theformation of only a small number of coarse MnS particles, which do nothave a deleterious effect. It is further known that the ratio ofmanganese to sulfur is correlative to the quality of the hot-rolledstrip edges in terms of the occurrence of edge crack. This ratio shouldtherefore be at least Mn/S>6, more preferably >20.

In the overall alloy considered here, tin is present elementally andsegregated at boundary surfaces. The presence of Sn in a concentrationof up to 0.150 wt %, but ideally in a range of 0.060-0.100 wt %, has abeneficial effect on the process of secondary recrystallization. Anadditional permanent slight impedance of grain boundary movement resultsoverall in sharper selection and therefore in greater texture sharpnessin the finished material. However, increased Sn contents can impair theadhesion of the insulating layer applied at the end of the productionprocess.

Copper is an element that in most cases becomes a steel admixturethrough the addition of scrap metal. Copper is known to segregate atboundary surfaces, and can therefore prevent the above-describeddamaging secondary effects of tin. For this reason, at least as muchcopper should be present in the alloy as tin, but ideally the ratio ofcopper (Cu) to tin (Sn) should be equal to 2. Copper also forms Cuprecipitations (Cu clusters), which likewise contribute to grain growthinhibition. In practice, Cu contents of up to 0.300 wt % exhibit nodisadvantages, with the ideal Cu content ranging from 0.150 to 0.250 wt%.

Aluminum is the main carrier of grain growth inhibition, and is based onthe acid soluble proportion of the aluminum. (The remainder is aluminaAl₂O₃). To adjust the effect of the inhibitor phase correctly, the Alcontent should be between 0.020 and 0.040 wt %, ideally between 0.026and 0.031 wt %.

Nitrogen, together with the acid soluble aluminum, acts as an inhibitorby way of the finely dispersed precipitation of AlN particles. Nitrogenis supplied to the strip in two ways: via an unavoidable proportionexisting in the composition of the smelt, and via the nitriding processduring annealing of the cold-rolled strip. In order to have as much freealuminum as bonded AlN (in number of atoms) available for the nitridingprocess, the N content of the smelt cannot exceed 25% of the Al content,but should range from 50 to 90 ppm and therefore below 100 ppm.

In addition to iron and unavoidable impurities, additional alloyelements, such as chromium, molybdenum, vanadium, nickel and others, mayalso be contained. Oxygen and boron contents must definitely be adjustedto values of less than 5 ppm. (Oxygen forms oxides, which as particlesdiminish the magnetic properties. Boron produces extreme brittleness andmust be avoided wherever possible.)

The method and manner of smelt production, e.g., the type and frequencyof secondary metallurgical treatments, is not important as long as thedesired alloy constituents can be prepared with reproducible precision.In particular, the secondary metallurgical treatment of the smelt shouldbe such that the addition of calcium to improve pourability is highlylimited. This is because calcium causes precipitations which must beavoided in principle for magnetic reasons.

The smelt is cast to form a strand at a maximum superheating temperatureof 40 K, ideally less than 20 K, and optimally less than 12 K, in eachcase referred to the liquidus temperature, which for the steel alloyconsidered here is very close to 1,493° C. Casting at a temperature justabove the liquidus point will result in an advantageously homogeneoussolidification structure with a high globulitic primary microstructureratio. However, with all of the above, production reliability must takepriority, in which too great a decrease in the superheating temperatureis associated with the risk of premature solidification. The strand iscast without exposure to inert gas, and conventional and usualprotection is provided during casting the strand in the tundish and themold.

The Liquid Core Reduction (LCR) casting method is also used, i.e.,casting is carried out into a metal mold having a thickness of 80-120mm, for example, after which the strand, which has not yet fullysolidified and still has a liquid core, is reduced by adjusting thesegments, preferably the first two segments, to a lower thickness rangeof between 50 and 120 mm, preferably 50-90 mm, in particular, 65-85 mm.In this manner, the more critical conditions which can occur during thinslab continuous casting as compared with the previously customary thickslab continuous casting are mitigated. Furthermore, this methodfacilitates casting at a lower superheating temperature. The vertical,rectilinear arrangement used during continuous casting over the entiremetallurgical length is advantageous for ensuring a high degree ofmetallurgical cleanliness. The fully solidified strand is bent to thehorizontal position at temperatures above 1,100° C., which has afavorable impact on the homogeneity of the inner microstructure.

The resulting strand is separated into individual thin slabs bycross-cutting, and is through-heated homogeneously in a compensatingfurnace to a maximum temperature of 1,250° C., but at least to atemperature that will allow the softened thin slab to be further workedin a reliable process. The time required for through-heating can bebetween 15 and 60 minutes.

Before the thin slab, which has undergone a first homogenizationannealing step in the continuous furnace, is hot rolled, it passeswithin a second homogenization step through a high-frequency inductionheating device, which is situated immediately upstream of the hotworking line and in which the thin slab is heated up to a temperaturebetween 1.300° C. and 1.380° C., preferably between 1.355° C. and 1.370°C., particularly up to 1.360° C. This inductor is ideally designed to becapable of raising the temperature of a thin slab measuring 60-90 mm inthickness, for example, and typically 1,000-1,300 mm in width by 150-300K as it is being advanced lengthwise into the hot working line at atypical infeed rate of less than 1 m/s. The structure of the inductiondevice is designed with respect to its electrical specifications(particularly frequency) such that uniform through-heating (skin depth)up to the core can be achieved.

An induction heating device of this type offers several technicaladvantages:

For one, this technical option gives the hot working process substantialthermomechanical degrees of freedom and therefore enormous flexibilityin designing the hot working/temperature/time process.

For another, it offers the technical option of selecting thecompensation temperature for the thin slabs as advantageously low, forexample, around 1,150° C., so that the thin slabs can thereafter beheated individually to any desired initial hot rolling temperature, upto approximately 1,380° C. In addition to the tremendous gain inlogistical flexibility in production, this allows a substantial savingsof energy in the large compensating furnace. It is also possible tooptimize the technology for the roller hearth of the compensatingfurnace. For example, at an appropriate, constant compensatingtemperature that is not overly high, water-cooled furnace rollers can bedispensed with, and in their place, simpler, uncooled rollers can beused. Substantial amounts of energy are saved as a result, since nothermal energy must be discharged to the exterior unused due to thewater cooling of the rollers.

For grain-oriented electrical steel strip, there is a further option interms of process engineering, which will result in improvements inmagnetic product characteristics and the homogenization thereof. Asstated above, the maximum through-heating temperature for thin slabs istechnically limited due to the limited high temperature strength ofthese formats. As a result, only acquired inhibition can be achieved.Now, with this new option of heating the thin slab material totemperatures up to 1,380° C. for several seconds, an inherent partialinhibition based on MnS and AlN can be achieved. The unavoidably lowhigh temperature strength at such high temperatures is non-problematicin this case, since the technical configuration of the materialconveyance system can be designed such that each of the slabs is pickedup and transported from the first stand of the hot working stage. Theinherent inhibition that can be produced in this manner is notsufficient to provide the total inhibition required in terms of thecomplete single cold-rolling process. Therefore, acquired inhibitionmust also be added, which is produced by nitriding the strip which hasbeen cold rolled to its usable thickness. But the main advantage overthe prior art is that, due to the inherent partial inhibition formedduring hot rolling, the microstructure of the strip, which is stabilizedon its processing path, which comprises hot-rolled strip annealing priorto cold rolling, and recrystallization, decarburization and nitridationannealing of the strip that has been cold-rolled to its usablethickness, prevents any parasitic grain growth.

Immediately after the inductive heating of the thin slab, the slab ishot rolled in the linear hot rolling stage to a hot strip thicknessranging from 1.80 to 3.0 mm, preferably from 1.8 mm to 2.5 mm. Based onthe overall temperature curve, the initial rolling temperature isgenerally substantially higher than 1,200° C. This ensures that fullrecrystallization of the hot worked cast structure will take place afterthe first, and at the latest after the second hot working pass. The highinitial rolling temperature likewise ensures maintenance of a safe finalrolling speed at the required high final rolling temperatures ofgenerally >950° C. In the present case, the maximum speed at which thesteel strip can be safely transported to reeling is 12 m/s. By selectingthe proper final temperature of the thin slab following inductionheating, the actual speeds can be reduced to 7.5 m/s, thereby decreasingthe risk of roller breaks and increasing yield as a result.

The hot rolled steel strip is subjected to hot rolled strip annealing,which is carried out for 180-300 seconds, typically 240 seconds, attemperatures of 950-1,150° C. Particularly important in hot rolled stripannealing is the rapid quenching of the steel strip that has just beenannealed, at a cooling rate of >30 K/s, preferably >40 K/s, andparticularly >45 K/s, ordinarily by means of water-injection nozzles athigh water pressure. For one thing, hot rolled strip annealing fulfillsthe function of microstructure homogenization. However, the regions ofthe hot rolled strip close to the surface, in which the Goss texture isalready present due to the shear ratios during hot working, are madesomewhat more coarse, which is advantageous in principle for theformation of the Goss texture in the subsequent cold rolling process.Moreover, the rapid cooling causes a finely dispersed carbideprecipitation. In the subsequent cold rolling process, this leads tointensified strain-hardening and therefore to energy being introducedinto the matrix. Immediately after water quenching, the surface of thehot-rolled strip is freed of the annealing scale by customary descalingand pickling techniques.

The hot-rolled strip annealing is followed by cold rolling, whichinvolves a single step of rolling to the finished strip thickness;however, this step is carried out in several successive passes. Thestandard nominal thicknesses for grain-oriented electrical steel stripare 0.35 mm, 0.30 mm, 0.27 mm, 0.23 mm and 0.18 mm. In this process,cold rolling must be performed such that the processing heat from coldwork that is introduced into the strip during the final 3 passes willact on the strip for long enough (at least 5 minutes) for the dissolvedcarbon content to settle in the dislocations which are heavily inducedduring plastic deformation, resulting in a further increase indeformation energy in the microstructure when cold rolling continues(known as the “Cottrell effect”). Although with conventional deep-drawnsteels, for example, the Cottrell effect is undesirable, forgrain-oriented electrical steel strip it is necessary in order to obtainthe most fine-grained and homogeneous microstructure followingrecrystallization, which also provides the best conditions for magneticcharacteristics. Ideally, such conditions are provided on a reversiblestand. The dimensions of this processing heat must be properlycontrolled during production. This refers the so-called “agingtemperature”. This temperature can be controlled by regularly placing acontact thermometer on the edge of the strip after each rolling pass, orcan be electronically detected continuously during cold rolling usingtechnical devices or equipment appropriate for this purpose. Thistemperature must be within a range of 180° C.-260° C., at least betweenone of the last 3 cold-rolling passes, and is typically 220° C. If atemperature of 180° C. is not achieved for a protracted period, theabove-described aging effect will be insufficient, and fluctuations inmagnetic quality will result. However, if this temperature reacheslevels above 260° C., oxide layers may form on the surface (“bluing”),which can lead to inhomogeneous gas reactions in the subsequentdecarburization and nitridation annealing.

The cold-rolled strip must be recrystallized in order to give it theproper crystallographic texture from which the secondaryrecrystallization can be optimally achieved. In principle, it isadvantageous here to choose the highest possible heating rate in orderto minimize the portion of recovery of the cold-rolled microstructurethat occurs prior to recrystallization. Normal heating rates of 20-40K/s, such as are possible in a conventional continuous annealing furnaceequipped with gas-fired jet burners, are sufficient for this. However,it is advantageous to increase the heating rate to levels of several 100K/s using an inductive or other type of rapid heating apparatus, forexample.

Recrystallization continuous annealing is known in the art.

Finally, the strip must be decarburized, with the carbon being reducedto residual levels of less than 30 ppm. This is important to prevent anycarbide from forming in the finished product, which would allow themagnetization losses to increase dramatically (magnetic aging). 30 ppmis the upper solubility limit for carbon in the ferritic matrix in thealloy being considered here, with approximately 3 wt % (2.5-4 wt %)silicon. Decarburization is carried out simultaneously withrecrystallization. The temperature of this annealing ranges from 820 to890° C., ideally 840-850° C., in which the strip surface-gas reaction ismost effective. Based on initial carbon content and strip thickness,annealing times of various lengths are required for decarburization,with a maximum time of 150 seconds, but typically of less than 100seconds. For the desired decarburizing strip surface-gas reaction, amoist annealing atmosphere containing hydrogen, nitrogen and water vaporis required. These constituents may be varied within wide limits, aslong as the oxidation potential remains suitably adjusted. This is thecase when the partial pressure ratio of water vapor to hydrogen pH₂O/pH₂is within a range of 0.30 to 0.60, preferably between 0.35 and 0.46.

The recrystallized and decarburized cold-rolled strip is then nitrided,in order to form the acquired inhibitor phase. This can be carried outat various temperatures ranging from 850 to 920° C., wherein a maximumaction time of 50 seconds, generally 15-40 seconds, in particular,typically approximately 30 seconds should be used. In this case, theannealing atmosphere comprises a mixture of hydrogen, nitrogen, watervapor and ammonia, in which a partial pressure ratio pH₂O/pH₂ rangingfrom 0.02 to 0.08, in particular, 0.03-0.07, is established. Theproportion of gaseous ammonia NH₃ in the total gas volume can range fromat least 2 wt % to a maximum of 12 wt %. These proportions are based onthe detailed structural conditions in the nitriding part, for example,the technical design of the infeed tuyere stocks, the distance thereoffrom the strip, and the infeed pressure, and must therefore beindividually optimized based on the object in question. Nitriding ingeneral is a multi-stage process. During the partial annealing treatmentdescribed thus far, the nitrogen has been first injected into a layervery near the surface, so that the total nitrogen content in the stripat this point is at least 180 ppm, preferably at least 350 ppm. In theinitial phase of the subsequent high-temperature annealing in abell-type furnace, which is used to perform secondary recrystallization,the nitrogen is spread by diffusion over the entire strip thickness andcombines with the aluminum present there locally to form AlN particles,which complete the already existing inherent partial inhibition.

This process of nitridation is a highly sensitive and failure-pronesurface-gas reaction, which can, in principle, lead to an inhomogeneousconfiguration. One problem with this process is that preceding it, thedecarburization annealing is necessarily performed under heavilyoxidative (moist) gas conditions, whereas nitridation is optimallycarried out in a drier annealing atmosphere with lower potential foroxidation. The highly oxidative decarburization annealing can thereforeform variously compact or locally inhomogeneous oxidative barrier layersthat will impede the subsequent nitridation. To combat this problem, anintermediate reduction annealing phase (intermediate reduction zone) isinterposed, in order to correct any local superoxidation that may haveformed during the immediately preceding decarburization annealing, sothat the nitridation treatment to follow directly can be performedhomogeneously and reproducibly. This intermediate reduction annealingphase should therefore be carried out under the same annealingatmosphere as the decarburization annealing immediately preceding it,but at a reduced partial pressure ratio pH₂O/pH₂ of <0.10, or ideally<0.05. The intermediate reduction annealing phase lasts a maximum of 40seconds, preferably 10 to 20 seconds. The temperature ranges from820-890° C. and should ideally be approximately centered between theoptimized and selected temperature levels for decarburizationnitridation treatment, which will simplify the process in terms ofsystems engineering.

At the primary recrystalization, decarburization, and nitridationannealing, neither texture ratios nor oxygen content are controlled.Rather, the related values are determined automatically by control ofrespective system and process. There is not any central and/orregulating system for controlling the texture ratios or the oxygencontent. A cold-rolled strip is obtained, which primary recrystallizedgrains have a circle equivalent mean size (diameter) between 22 μm and25 μm.

The steel strip that has been recrystallized, decarburized and nitridedin this manner is then coated with a non-stick layer (annealingseparator), before it can be further processed, wound to form a coil, byhigh-temperature annealing in a bell-type furnace. The non-stick coatingis applied to the steel strip as an aqueous slurry of MgO powder indemineralized water. Here, it is important to minimize the pick-up ofcrystal water in the MgO, for which purpose measures such as minimizingthe period of contact between the MgO and the water, cooling the entireMgO slurry and coating system and the cold-rolled strip itself to 4° C.,and rapidly drying the coating offer possible options.

The formation of the Goss texture by the process of secondary graingrowth is carried out by means of traditional high-temperature annealingin a bell-type furnace. The coils, which have been coated with anon-stick layer, are placed on highly heat-resistant steel platesthrough which the annealing gas is directed, and are encompassed byprotective hoods. The heating hoods are then placed over these, and areeither fired with gas or electrically heated. Once the entire annealingassembly has been flushed with dry nitrogen gas at the start of eachannealing pass, a rapid heating to 400° C. is performed, followed byslow heating at approximately 15-20 K/h, up to a holding temperature of1,190-1,210° C. In this process, an intermediate holding stage lasting 5to 10 hours can advantageously be introduced, at a temperature rangingfrom 600-700° C., particularly at 650° C., which serves to compensatefor temperature gradients of heavy and thermally sluggish coils. Duringthis slow heating phase, the protective hoods are supplied with amixture of dry nitrogen and hydrogen. Dry annealing gas is particularlyimportant in this case, because any water vapor fractions will disruptthe sensitive process of texture formation. However, a certain increasein humidity is unavoidable beyond a temperature of 400° C., as a resultof evaporating crystal water, which is unavoidable in small quantities,from the MgO non-stick layer. That is why it is so important to minimizethe pick-up of crystal water by the above-described measures.

With respect to the composition of the annealing gas during the heatingphase, an annealing atmosphere having a strongly predominant nitrogenproportion of up to 90 wt % N₂ is used. Such an excess of nitrogenallows the period of action of the AlN inhibitor phase to be extendedsomewhat, because the decomposition of AlN and the removal of thereleased nitrogen are delayed somewhat.

Once the holding temperature is reached, the gas supply is switched to100% hydrogen, and is maintained for at least 20 hours at 1,190-1,210°C. To optimize the holding time and the holding temperature, the totalpurification of sulfur and nitrogen must be ensured, and the formationof edge defects on the stand edge of the coils (bottom buckles) must beminimized.

When this high-temperature holding time has expired, the resultingfinished steel strip is cooled to ambient temperature. During thisprocess, feeding with 100% hydrogen gas is initially maintained, inorder to avoid any nitrogen pick-up. However, as soon as the temperaturein the coils drops below approximately 600° C., the annealing atmosphereis switched to 100% dry nitrogen. As soon as the temperature drops below400° C., the heating hoods can be raised, and when it drops belowapproximately 100° C. the protective hoods can also be raised.

Following the high-temperature annealing in a bell-type furnace, thesecondary recrystallized finished steel strip is mechanically cleaned ofexcess residual MgO (using water and rotating brushes), thenadvantageously pickled in a bath with phosphoric acid, and immediatelythereafter and directly downstream, said strip is fed to a continuousannealing line, where it is stress-relief annealed. As is known inpractice, the moist, coated steel strip is usually suspended in a longloop in the intake region of a continuous annealing line. In thisfurnace region, the steel strip is heated with high heating power, inwhich process the insulating coating is also fully set and dried. Onlythen is the steel strip permitted to touch the first furnace transportroller. The annealing atmosphere that is used is non-critical, as is theheating speed, however, the maximum temperature that is reached must bebetween 840 and 880° C., and is ideally 860° C., in order to remove anymechanical stresses and to produce a steel strip that is evenlydirected. If the temperature drops too far below this level, the desiredeffect will not be produced. If it is too far above this level, theinsulating coating can sustain damage. However, it is particularlyimportant for the cooling in which the steel strip is brought back tothe ambient temperature to be as homogeneous as possible. This isusually achieved by using ventilators over a relatively long coolingpass.

In the outlet region of this last annealing line in the overall processof producing grain-oriented electrical steel strip, the finished productis delivered, and can be evaluated and certified on the basis ofquality-relevant criteria.

The grain-oriented electrical steel strip that has been processed toform the final product can also be optionally subjected to a subsequentmagnetic domain refinement, which can decrease the magnetization lossesby an additional 12-20%. Such a device for domain refinement can beinstalled in the outlet part of the final insulation/stress-reliefannealing system or can optionally be performed offline.

The procedure for one embodiment example is as follows:

A steel smelt having a chemical composition of

3.230 wt % Si

0.058 wt % C

0.168 wt % Mn

0.206 wt % Cu

0.003 wt % S

0.030 wt % Al (acid soluble)

0.088 wt % Sn

0.003 wt % N

0.087 wt % Cr

0.001 wt % Ti

0.029 wt % P

0.085 wt % Ni

and small, unavoidable quantities of impurities is cast in a metal moldto a casting thickness of 85 mm using the thin slab continuous castingtechnique, and is formed by the “liquid core reduction” method (inthickness reduction according to the “liquid core reduction” method, thestrand thickness is reduced just below the metal mold while the interiorof the strand has a liquid core. Also possible is the so-called “softreduction” method, in which a selective thickness reduction of the caststrand is (first) performed at the solidification point close to finalsolidification), without exposure of the strand to inert gas, to astrand having a thickness of 65 mm to 85 mm (the latter thickness beingachieved without liquid core reduction) and a width of 1,100 mm to 1,250mm.

Following a subsequent controlled cooling, the strand that is producedreaches a temperature behind the metallurgical length of 1,190° C., atwhich the strand is bent from vertical to horizontal and is then dividedcrosswise into individual slabs. Thus the slabs are produced by means ofthe so-called thin slab technology. The slabs are then subjected to 20minutes of homogenization annealing at 1,150° C., which is finished by ahigh-temperature treatment. For that purpose the slabs are guidedthrough an electrically powered continuous induction heating device,immediately prior to the first hot rolling pass, and are brought by saiddevice to a temperature of 1,370° C. at least for a short time ofseveral seconds. The (thin) slabs then pass through a high-pressuredescaling device followed by a hot-rolling treatment in form of ahot-rolling process in a rolling mill.

The first hot working pass is carried out approximately 10 s afterleaving the inductor or the induction heating device, at a temperatureof approximately 1,280° C.

In the embodiment example, the thin slab is hot rolled to a hot strip ina hot-rolling train comprising 6 stands, wherein after leaving the laststand, each hot-rolled strip has a thickness of 2.30 mm.

Upon completion of hot-rolling treatment, the hot-rolled strip that isproduced from the thin slab has a final rolling temperature of 930° C.It then passes, after approximately 5 s, through a laminar cooling path,before being wound at a reeling temperature of approximately 580° C. toa reel to form a coil.

The hot-rolled strip produced/generated in this manner will later be fedto a cold rolling process.

The cold rolling process begins with trimming the rough hot strip edgesof the hot-rolled strip, after which it is fed to a continuous annealingprocess, by means of which it is annealed over a period of 220 s undernon-oxidizing conditions (gas atmosphere with 95 wt % dry N₂ and 5 wt %H₂) at a maximum temperature of between 920° C. and 1,150° C., inparticular at a temperature of 950° C. or 1,050° C. or 1,120° C.

Immediately after the annealing process, within 5 seconds of leaving thefurnace that was used for the annealing process, the annealed hot-rolledstrip is subjected to a high-pressure water spraying and a cooling rateranging from 28 K/s to 52 K/s, in particular, defined cooling rates of52 K/s or 45 K/s or 38 K/s or 28 K/s.

The hot-rolled strip that has been annealed and quenched in this manneris then pickled, wherein the surface scales are broken up and aredissolved by pickling chemicals like hydrochloric acid.

The hot-rolled strip which is now prepared for cold rolling is then fedto a cold rolling process, in which it is cold rolled, in a reversiblecold-rolling stand in a single process comprising multiple passes, to anominal usable thickness ranging from 0.23 mm to 0.30 mm, in particular,to the standard nominal thickness of 0.23 mm or 0.27 mm or 0.30 mm. Inthis process, the strip temperature that results from the working heatis adjusted and controlled such that during the second to the last coldrolling pass, a temperature of 235° C., or a maximum of 260° C., isestablished, and the strip is exposed to this temperature for 10minutes, but at least for 5 minutes.

The strip which has been cold rolled to its usable thickness is thensubjected in a furnace to recrystallization, decarburization, thenintermediate reduction, and finally nitridation continuous beltannealing treatment, the procedure for which is as follows:

The cold-rolled steel strip is first heated in the furnace at an averageheating rate of 30 K/s to a holding temperature of 850° C. and isannealed for a maximum of 150 seconds at this temperature, wherein thegaseous annealing atmosphere in the furnace consists of a moist mixtureof 60 wt % N₂ and 40 wt % H₂, at a saturation temperature of 54° C., sothat a water vapor/hydrogen partial pressure ratio pH₂O/pH₂=0.44 isestablished.

In the embodiment example, following an annealing step under theaforementioned conditions in the same furnace lasting 95 seconds, thestrip then reaches a separate zone in which the annealing gas orannealing atmosphere has the same composition, but a differentsaturation temperature of only 10 to 16° C., which corresponds to awater vapor/hydrogen partial pressure ratio pH₂O/pH₂ of 0.03 to 0.05.The temperature in this furnace zone is 880° C.

Once the strip has been exposed to these conditions for a period of 20seconds, it reaches a third separate zone in the furnace, in which it isthen annealed at a temperature of 910° C. for a period of 30 seconds,wherein the gaseous annealing atmosphere at this point in the furnaceconsists of a mixture of 30 wt % N₂ and 70 wt % H₂ with a saturationtemperature of 26° C., so that a water vapor/hydrogen partial pressureratio pH₂O/pH₂ of 0.05 is established. A quantity of 7 wt % ammonia(NH₃) is added to the gaseous annealing atmosphere (annealing gas);however, this quantity is not admixed with the annealing gas alreadypresent. Instead, the previously cooled NH₃ is blown immediately anddirectly onto the surfaces of the strip via (special) tuyere stocks,which are located above and below the strip to be treated. The annealedcold-rolled strip then has a total nitrogen content of 320 ppm and aprimary recrystallized grain having a circle equivalent mean size(diameter) of 24 μm.

A coating of an annealing separator (non-stick layer) consisting of MgOwith additives of 5 wt % TiO₂, 0.5 wt % Na2B4O7 and 0.05 wt % MgCl2(quantities referred to the quantity of MgO) is then applied to theannealed cold-rolled strip which has been treated and prepared in thismanner. Both the steel strip and the annealing separator in the form ofan aqueous anti-adhesion slurry are cooled to 4° C. prior to coating.Immediately after the coating with the annealing separator (non-sticklayer), the two opposing large-area steel strip surfaces (surface areas)are dried using intensive infrared radiation. The cold-rolled strip isthen wound onto a reel to form a coil, tilted to a position in which thecoil axis is vertical, and delivered in this position.

To obtain a grain-oriented electrical steel strip having a Goss texturefor use in transformers, which is characterized by a particularly sharp{110}<001> texture (Miller indices) and which has a slight magnetizationdirection parallel to the rolling direction, a Goss texture is thenformed in a secondary recrystallization process, for which purpose thecoils are annealed in a high-temperature, bell-type annealing furnace,in which a heating rate of 20 K/h is established. The heating phase isinterrupted by a holding stage at 650° C., during which the temperatureis maintained for a period of 5 hours for the purpose of temperaturecompensation. Heating is then continued as before, until a temperatureof 1,200° C. is reached. Throughout this time, a dry gas consisting of75 wt % N₂ and 25 wt % H₂ flows through the annealing hood. Thetemperature of 1,200° C. represents the holding temperature at which,when reached, the gas atmosphere prevailing in the annealing hood isswitched to 100% dry hydrogen. The coils are annealed for 24 hours atthis high-temperature holding stage of 1,200° C. This is followed bygradual cooling to the ambient temperature, in which, when thetemperature drops below 600° C., the gas atmosphere in the annealinghood is switched to 100% N₂. During the heating phase, but preferablyduring the entire secondary recrystallization annealing process in thehigh-temperature bell-type annealing furnace, the quantity of Nitrogen(N₂), in terms of atomic percent, in the gaseous annealing atmosphere isgreater than the quantity of hydrogen (H₂), in terms of atomic percent.Following this secondary recrystallization annealing, the grain-orientedelectrical steel strip with Goss texture is finished.

Once the finished steel strip obtained in this manner has cooled to theambient temperature, it is washed, pickled in phosphoric acid, coatedwith a liquid phosphating agent and finally continuously stress-reliefannealed at a maximum temperature of 860° C. and then uniformly cooled.

The grain-oriented electrical steel strip produced in this manner hasvery good magnetic characteristics in the range of conventional HGO(High permeability Grain Oriented) material. The remagnetization loss at50 Hz and 1.7 T modulation for such a steel strip having a finishedsteel strip nominal thickness/usable nominal thickness of 0.23 mm is0.79 W/kg with a polarization of 1.93 T at a field strength of 800 A/m.

What is claimed is:
 1. A process for producing grain-oriented electricalsteel strip by means of thin slab continuous casting, comprising thefollowing process steps: a) smelting a steel with a smelt which, inparticular after secondary metallurgical treatment, contains, inaddition to iron (Fe) and unavoidable impurities, Si: 2.50-4.00 wt %, C:0.030-0.100 wt %, Mn: 0.060-0.300 wt %, Cu: 0.100-0.300 wt %, Alsl:0.020-0.040 wt % Sn: 0.050-0.150 wt % S: <100 ppm, N: <100 ppm, and oneor more elements from the group comprising Cr, V, Ni, Mo and Nb, b)continuously casting the smelt by thin slab continuous casting, withoutexposure of the strand to inert gas, to form a strand having a thicknessof 50-120 mm, and dividing the strand into thin slabs, c) carrying out ahomogenization annealing comprising the steps of c′) heating the thinslabs, preferably in a linear furnace, to a temperature above 1,050° C.and subjecting the slabs to an annealing at a maximum of 1,250° C.,preferably at a maximum of 1,200° C., in particular, at a maximum of1,150° C., and d) feeding the thin slabs to an induction heating device,in particular, a high-frequency induction heating device, in which thethin slabs particularly while passing are immediately prior to the firsthot-rolling pass heated for several seconds up to a temperature of1,350° C.-1,380° C., preferably of 1.355° C.-1.370° C., particularly of1.36,0° C. that is above the previous homogenization temperature ofprocess step c′), e) continuously hot rolling the thin slabs in apreferably linear, multiple-stand hot-rolling train to form a hot striphaving a thickness of 1.8 mm-3.0 mm, f) cooling and reeling thehot-rolled strip at a reeling temperature of less than 650° C. to form acoil, g) annealing the hot-rolled strip, after reeling and prior to asubsequent cold rolling step, at a temperature of between 920° C. and1,150° C., h) cold rolling the hot-rolled strip, preferably on areversible stand, in a single process step in more than three passes, tocold-rolled strip having a final thickness of 0.15 mm-0.40 mm, i)subjecting the resulting cold-rolled strip to recrystallization,decarburization and nitridation annealing, j) applying an annealingseparator (non-stick layer), in particular containing primarily MgO, tothe strip surface of the cold-rolled strip which has beenrecrystallization, decarburization and nitridation annealed, k)subjecting the cold-rolled strip which has been coated with theannealing separator to secondary recrystallization annealing byhigh-temperature annealing in a bell-type furnace at a temperatureof >1,150° C., forming a finished steel strip having a pronounced Gosstexture, l) coating the finished steel strip which has undergonesecondary recrystallization annealing with an electrically insulatinglayer, and then stress-free annealing or stress-relief annealing thecoated finished steel strip, wherein the recrystallization,decarburization and nitridation annealing of the cold-rolled strip inprocess step i) comprises a decarburization annealing phase, which iscarried out at a strip temperature ranging from 820° C.-890° C. for amaximum period of 150 seconds, with a gaseous annealing atmosphere, inparticular moist, which contains nitrogen (N₂) and hydrogen (H₂) andacts on the cold-rolled strip, and which has a water vapor/hydrogenpartial pressure ratio pH₂O/pH₂ of 0.30 to 0.60, and comprises asubsequent nitridation annealing phase, which is carried out at atemperature ranging from 850° C.-920° C. for a maximum period of 50seconds, with a gaseous annealing atmosphere which contains nitrogen(N₂) and hydrogen (H₂) and acts on the cold-rolled strip, and which hasa water vapor/hydrogen partial pressure ratio pH₂O/pH₂ of 0.03 to 0.07,and comprises an intermediate reduction annealing phase, which iscarried out between the decarburization annealing phase and thenitridation annealing phase and is carried out at a temperature rangingfrom 820° C.-890° C. for a maximum period of 40 seconds, with a gaseousannealing atmosphere, in particular dry, which contains nitrogen (N₂)and hydrogen (H₂) and acts on the cold-rolled strip, and which has awater vapor/hydrogen partial pressure ratio pH₂O/pH₂ of less than 0.10and wherein a cold-rolled strip is obtained, which primaryrecrystallized grains have a circle equivalent mean size (diameter)between 22 μm and 25 μm.
 2. The process according to claim 1,characterized in that at least 2 wt % to a maximum of 12 wt % ammonia(NH₃), in particular, referred to the total gas flow rate, is addedseparately to the annealing atmosphere during the nitridation annealingphase in process step i), and the ammonia is blown onto the twoopposing, large-area strip surfaces of the cold-rolled strip.
 3. Theprocess according to claim 1, characterized in that, during theannealing in process step i), which comprises the decarburizationannealing phase, the intermediate reduction annealing phase and thenitridation annealing phase, the cold-rolled strip is annealed such thatafter annealing, the cold-rolled strip has a total nitrogen content ofat least 180 ppm, preferably at least 250 ppm, in particular, at least300 ppm.
 4. The process according to claim 1, characterized in that, atthe start of recrystallization annealing in process step i), thecold-rolled strip is heated at a heating rate of more than 100 K/s. 5.The process according to claim 1, characterized in that in the smelt inprocess step a), the ratio of manganese (Mn) to sulfur (S) is greaterthan 6, preferably greater than 20, and the ratio of aluminum (Al) tonitrogen (N) is greater than 4, preferably greater than
 10. 6. Theprocess according to claim 1, characterized in that during casting inprocess step b), the superheating temperature of the smelt duringcasting is less than 40 K, preferably less than 20 K, in particular,less than 12 K, and the reduction in the thickness of the strand iscarried out according to the “liquid core reduction” method, just belowthe metal mold, while the interior of the strand has a liquid core. 7.The process according to claim 1, characterized in that the hot rollingin process step e) is carried out at an initial rolling temperatureduring the first working pass of greater than 1,150° C., preferablygreater than 1,200° C., a final rolling temperature ranging from 850°C.-980° C. and a final rolling speed of less than 12 m/s, preferablyless than 10 m/s.
 8. The process according to claim 1, characterized inthat, during annealing of the hot-rolled strip in process step g), theannealed hot-rolled strip is quenched after annealing at a cooling rateof more than 25 K/s, preferably more than 30 K/s, in particular, morethan 40 K/s, particularly preferably a cooling rate ranging from 25K/s-52 K/s.
 9. The process according to claim 1, characterized in thatthe working during the cold-rolling of the hot-rolled strip in processstep h) is carried out such, that during at least one or more of thefinal three passes, the hot-rolled strip reaches a temperature of atleast 180° C. to a maximum of 260° C., resulting from the processingheat produced during rolling, for a period of at least five minutes. 10.The process according to claim 1, characterized in that the cold rollingin process step h) is carried out in two stages, wherein the hot-rolledstrip is pickled in a pickling step prior to the first cold-rollingstage, and upon completion of the first cold-rolling stage, thehot-rolled steel is annealed according to process step g).
 11. Theprocess according to claim 10, characterized in that in the secondcold-rolling stage, the thickness of the hot-rolled strip is reduced byat least 85%.
 12. The process according to claim 1, characterized inthat the secondary recrystallization annealing in process step k) iscarried out such that, during the heating phase of the high-temperatureannealing in a bell-type furnace, the quantity of nitrogen (N₂), interms of atomic percent, in the gaseous annealing atmosphere is greaterthan the quantity of hydrogen (H₂), in terms of atomic percent.
 13. Theprocess according to claim 1, characterized in that, following processstep l), a process step that effects a magnetic domain refinement of thecoated finished steel strip is carried out, particularly optionally. 14.A grain-oriented electrical steel strip obtained by a process comprisingthe following steps: a) smelting a steel with a smelt which, inparticular after secondary metallurgical treatment, contains, inaddition to iron (Fe) and unavoidable impurities, Si: 2.50-4.00 wt %, C:0.030-0.100 wt %, Mn: 0.060-0.300 wt %, Cu: 0.100-0.300 wt %, Alsl:0.020-0.040 wt % Sn: 0.050-0.150 wt % S: <100 ppm, N: <100 ppm, and oneor more elements from the group comprising Cr, V, Ni, Mo and Nb, b)continuously casting the smelt by thin slab continuous casting, withoutexposure of the strand to inert gas, to form a strand having a thicknessof 50-120 mm, and dividing the strand into thin slabs, c) carrying out ahomogenization annealing comprising the steps of c′) heating the thinslabs, preferably in a linear furnace, to a temperature above 1,050° C.and subjecting the slabs to an annealing at a maximum of 1,250° C.,preferably at a maximum of 1,200° C., in particular, at a maximum of1,150° C., and d) feeding the thin slabs to an induction heating device,in particular, a high-frequency induction heating device, in which thethin slabs particularly while passing are immediately prior to the firsthot-rolling pass heated for several seconds up to a temperature of1,350° C.-1,380° C., preferably of 1.355° C.-1.370° C., particularly of1.36,0° C. that is above the previous homogenization temperature ofprocess step c′), e) continuously hot rolling the thin slabs in apreferably linear, multiple-stand hot-rolling train to form a hot striphaving a thickness of 1.8 mm-3.0 mm, f) cooling and reeling thehot-rolled strip at a reeling temperature of less than 650° C. to form acoil, g) annealing the hot-rolled strip, after reeling and prior to asubsequent cold rolling step, at a temperature of between 920° C. and1,150° C., h) cold rolling the hot-rolled strip, preferably on areversible stand, in a single process step in more than three passes, tocold-rolled strip having a final thickness of 0.15 mm-0.40 mm, i)subjecting the resulting cold-rolled strip to recrystallization,decarburization and nitridation annealing, j) applying an annealingseparator (non-stick layer), in particular containing primarily MgO, tothe strip surface of the cold-rolled strip which has beenrecrystallization, decarburization and nitridation annealed, k)subjecting the cold-rolled strip which has been coated with theannealing separator to secondary recrystallization annealing byhigh-temperature annealing in a bell-type furnace at a temperatureof >1,150° C., forming a finished steel strip having a pronounced Gosstexture, l) coating the finished steel strip which has undergonesecondary recrystallization annealing with an electrically insulatinglayer, and then stress-free annealing or stress-relief annealing thecoated finished steel strip, wherein the recrystallization,decarburization and nitridation annealing of the cold-rolled strip inprocess step i) comprises a decarburization annealing phase, which iscarried out at a strip temperature ranging from 820° C.-890° C. for amaximum period of 150 seconds, with a gaseous annealing atmosphere, inparticular moist, which contains nitrogen (N₂) and hydrogen (H₂) andacts on the cold-rolled strip, and which has a water vapor/hydrogenpartial pressure ratio pH₂O/pH₂ of 0.30 to 0.60, and comprises asubsequent nitridation annealing phase, which is carried out at atemperature ranging from 850° C.-920° C. for a maximum period of 50seconds, with a gaseous annealing atmosphere which contains nitrogen(N₂) and hydrogen (H₂) and acts on the cold-rolled strip, and which hasa water vapor/hydrogen partial pressure ratio pH₂O/pH₂ of 0.03 to 0.07,and comprises an intermediate reduction annealing phase, which iscarried out between the decarburization annealing phase and thenitridation annealing phase and is carried out at a temperature rangingfrom 820° C.-890° C. for a maximum period of 40 seconds, with a gaseousannealing atmosphere, in particular dry, which contains nitrogen (N₂)and hydrogen (H₂) and acts on the cold-rolled strip, and which has awater vapor/hydrogen partial pressure ratio pH₂O/pH₂ of less than 0.10and wherein a cold-rolled strip is obtained, which primaryrecrystallized grains have a circle equivalent mean size (diameter)between 22 μm and 25 μm.